The term “RF energy” is used herein for electromagnetic energy in any part of the spectrum from LF to microwaves inclusive.
Applying RF energy to materials to produce heat, or some other desired effect, is a well-known technique used in both domestic and industrial processes.
FIG. 1 shows a typical arrangement of a well-known domestic microwave oven 100, which comprises a microwave generator. In a microwave oven 100, a magnetron 11 is powered through a transformer T1, a first end of a secondary winding 121 of the transformer T1 being connected via a capacitor C1 to a cathode 111 of the magnetron 11 and a second end of the secondary winding 121 of the transformer T1 being connected to an anode 112 of the magnetron 11. The magnetron anode 112 is usually grounded. A diode D1 is connected between the cathode and anode of the magnetron 11. A single phase mains voltage is applied through a switch S1 to a primary winding 123 of the transformer T1. On a positive half cycle of a single phase main supply, between times T1 and T2 on a first voltage plot 101 of FIG. 1, capacitor C1 charges up to +Vpk, substantially equal to a maximum positive voltage output from the secondary winding of transformer T1. On a negative half cycle of the single phase mains supply, between times T0 and T1, and between times T2 and T3 in plot 102 of FIG. 1, the capacitor voltage on C1 and a negative voltage from the secondary winding, having a maximum value −Vpk, are added to provide a variable voltage with a maximum value of −Vk and this results in a variable current flow into the magnetron, shown in plot 103, with a maximum value of Ipk and an average value Imean. The variable current is a clipped half sine wave and usually has a peak current Ipk to mean current Imean ratio of approximately three.
Transformer T1 usually incorporates a non-linear leakage reactance to provide a current shaping facility and a degree of regulation against mains voltage variations. A second secondary winding 122 is also usually provided in the transformer T1, to supply a magnetron heater 113, as shown in FIG. 1.
Power control of the RF generator is usually effected by closing switch S1 using a control system 13 connected thereto, typically by switching power on for a period of several seconds followed by switching power off for several seconds in repeated cycles.
Systems in this basic form are well known and behave satisfactorily for moderate powers, typically of up to 1.5 kW average electrical input with the magnetron delivering a pulse of RF power with a peak value of 3 kW and an average value 1 kW.
For very high powers, for example 50 kW average power, it is normal to use a three phase mains supply as a prime power source. An example of such a typical known high-power system 200 is shown in FIG. 2. A three phase mains supply is input via a breaker 20 to a mains transformer 22 and output to a 6 pulse rectifier 23. This arrangement produces a relatively smooth voltage with low ripple. However, as a magnetron 21 has a biased diode type of load, a small change in voltage due to a ripple voltage produces a much larger change in current. To produce steady current in the magnetron 21 a large choke 24 is used as a filter between a first output 231 of the 6 pulse rectifier 23 and a cathode K of the magnetron 21.
A starting voltage of the magnetron 21 is a function of a magnetic field strength applied thereto. By varying a magnetic field, the magnetron operating voltage can be raised or lowered, thus a power demand of the magnetron can be varied without any main power supply control features. The magnetic field is varied using a controllable power supply, such as an SCR controller 29, to vary a current supplied to a solenoid 28 arranged to apply the magnetic field to the magnetron 21. With high power magnetrons it is also usual to vary a voltage applied to a heater H of the magnetron 21, to optimise a temperature of the cathode K and this is carried out by a heater SCR controller 27 operating on a primary winding of a heater transformer 26, a secondary winding of the heater transformer being connected between the cathode K and the heater H of the magnetron 21.
With both the previously described standard known systems a number of variations are possible. For example, use of a single switched mode power supply (SMPS) is known in both low-power and high-power applications.
A disadvantage with the known arrangements is that should such an RF generator spark internally, a quantity of energy that can pass through the spark into the RF generator device is quite considerable and the risk of damage to the tube 11, 21 is high. Neither circuit is well suited to the rapid interruption of, or restoration of, power to the RF generator. This is because of a slow response of a contactor 51, 20 in interrupting power, and stored energy in a filter 24 and smoothing components associated with the rectification process.
A single switched mode power supply achieves ripple reduction with much smaller stored energy in the filters but the energy may still be significant and shutting down the system and restarting may still take several seconds thus resulting in an undesirable interruption of a process using the RF generator.
Most RF generators, and particularly magnetrons, produce optimum conversion of electrical energy to RF output energy over a small operating range of current and voltage. Thus a method of power control that maintains the RF generator at its optimum efficiency but still permits wide-ranging average power control to be obtained is desirable.
It is desirable that the power demand from the electrical utility is of good quality. The most desirable power factor is 1. This implies a load drawing power has characteristics of a linear resistor. With rectifiers, loosely described as any device that converts AC to DC, harmonic distortion of the mains is a well understood side effect and a gradually increasing problem for power utilities as a larger percentage of power is taken by electronic devices that result in a high harmonic content of the power supply. As an example, a commonly used 6 pulse 3 phase rectifier has a power factor of around 0.95 but produces around 25% harmonic distortion in the mains supply. A properly designed SMPS would have a power factor of 0.99 and additional harmonic distortion of the mains may be only 1% or 2%. A power supply for an RF generator thus requires a very good power factor (>0.98) and very low harmonic distortion, preferably of less than 4%. System 100, illustrated in FIG. 1, can be quite acceptable but could be improved. The high-power system 200 of FIG. 2 leaves a lot to be desired and under many situations does not meet regulatory requirements.
The system 100 of FIG. 1 provides pulses of energy but only as a by-product of its method of operation. A system that could provide RF energy input of an average power (Pave) but with a specified peak power (Ppk) can bring added benefits to a given process.
It is an object of the present invention at least to ameliorate the aforesaid disadvantages in the prior art.